In a known display, light of the three primary colors (red (R), green (G), and blue (B)) is emitted using one of the methods shown in FIGS. 7(a) to 7(c).
In the method shown in FIG. 7(a), a white luminescent medium 26 which emits white light is provided between a first electrode 22 and a second electrode 24, and light emitted from the white luminescent medium 26 is caused to pass through a red filter 28 which transmits only red light and blocks green light and blue light, a green filter 30 which transmits only green light and blocks red light and blue light, and a blue filter 32 which transmits only blue light and blocks red light and green light to obtain the RGB colors.
In the method shown in FIG. 7(b), a red luminescent medium 34 which emits red light, a green luminescent medium 36 which emits green light, and a blue luminescent medium 38 which emits blue light are disposed in parallel between the first electrode 22 and the second electrode 24 to obtain the RGB colors.
In the method shown in FIG. 7(c), the blue luminescent medium 38 is provided between the first electrode 22 and the second electrode 24, and light emitted from the blue luminescent medium 38 is caused to pass through a red fluorescent film 40 which converts the blue light into red light, a green fluorescent film 42 which converts the blue light into green light, and a blue filter 44 which adjusts the color of the light emitted from the blue luminescent medium to obtain the RGB colors.
The white luminescent medium 26 shown in FIG. 7(a) may be formed by disposing emitting layers of at least two colors in parallel between the first electrode 22 and the second electrode 24. The white luminescent medium 26 may also be formed by stacking emitting layers of at least two colors between the first electrode 22 and the second electrode 24.
However, these known technologies cannot realize a highly efficient emitting device which can be applied as a surface light source.
The inventors of the invention conducted extensive studies on the reasons why a highly efficient emitting device cannot be obtained. As a result, the inventors found a fundamental problem of an emitting device. Specifically, the intensity of light from the luminescent medium outcoupled in the front direction of the device is decreased due to high intensity of light components propagated in the direction parallel to the two electrodes and the total reflected components at the interface between a transparent substrate and air on the light-outcoupling side. It was found from measurement of the total quantity of light that the amount of loss reached 70 to 80%.
In an emitting device shown in FIG. 8, a first electrode 22, a luminescent medium 48, and a second electrode 24 are stacked in this order on a supporting substrate 46, for example. A luminescent material in the luminescent medium 48 emits light in all directions. Only the light emitted in a light-outcoupling direction A is outcoupled through the supporting substrate 46. Part of the light emitted in the light-outcoupling direction A is converted by a fluorescent film 50 into light having a different wavelength and then outcoupled.
However, the light emitted in the directions other than the light-outcoupling direction A, such as directions X and Y parallel to the supporting substrate 46, cannot be outcoupled from the device and utilized. Moreover, part of the light emitted in the light-outcoupling direction A is reflected (not shown) by the substrate 46 and the like and cannot be utilized. Therefore, such a device exhibits insufficient emission efficiency with the above-mentioned large amount of loss.
In view of the above circumstances, the invention was made and an object thereof is to provide a composite emitting device with a high efficiency.
In order to attain the object, the inventors found through extensive studies a novel device capable of emitting light with a high efficiency by utilizing the loss not outcoupled in the front direction.